1. Field of the Invention
The present invention relates to a method of controlling a thermal head.
2. Description of the Background Art
A thermal head, hitherto provided in a printer and the like, includes a plurality of resistors. Based on data for printing, each resistor is selectively energized to execute printing on sheet surface.
Further, the thermal head has hitherto been controlled in a manner as described below for realizing an image with a predetermined gradation on one line. Namely, one line period is constituted by an energizing period in which resistors are energized and a non-energizing period in which the resistors are not energized. The non-energizing period is provided after the energizing period. More specifically, in the one line period, the resistors are energized during the energizing period after the start of the one line period, and the non-energizing period is provided after the energizing period.
For example, when printing an image with relatively low gradation (dark-colored image) is required, resistors are energized from the line-start, and after a lapse of the relatively long energizing period, the relatively short non-energizing period is elapsed (line-end). On the other hand, when printing an image with relatively high gradation (lightly colored image) is required, the resistors are energized from the line-start, and after a lapse of the relatively short energizing period, the relatively long non-energizing period is elapsed (line-end).
As thus described, by adopting the method of controlling a thermal head in which the energizing period is provided from the line-start and the non-energizing period is provided after the energizing period for each line, it is possible to represent an image with a predetermined gradation on each line.
As background art documents relating to the above-mentioned conventional method of controlling a thermal head, there are, for example, Japanese Patent Application Laid-Open No. 2001-138561, Japanese Patent Application Laid-Open No. 8-142376 and Japanese Patent Application Laid-Open No. 10-305607.
With the conventional method of controlling a thermal head as described above, there has been a problem that when a moving speed of a thermal head (or moving speed of sheet) becomes high, the thermal head comes into a state where its thermal response does not follow the speed. Therefore, e.g., on a changing portion (border) from black to white (or white to black), the image sharpness is impaired.